The present disclosure relates to an anode for a galvanic cell and a galvanic cell having this anode.
Rechargeable galvanic cells, for example, battery cells having an anode containing lithium, have a very high available energy density or specific energy, in particular in comparison to battery cells based on nickel-metal hydride or lead-acid. Lithium-ion batteries may be used both in partially or completely electrically powered vehicles, electric vehicles or hybrid vehicles, and in computer technology, in particular in notebooks, smartphones, or tablet PCs. Lithium-ion cells have at least one positive electrode (cathode) and at least one negative electrode (anode), which are able to reversibly intercalate or again deintercalate lithium ions. Rechargeable batteries, also referred to as secondary batteries, are parallel or serial combinations of at least two individual electrically connected accumulators or battery cells. These batteries are also referred to as a battery pack or battery module.
Batteries having different chemical systems, such as lead-acid, nickel-metal hydride, and lithium-ion, are available on the market in various design forms, such as button cells, cylindrical cells, prismatic cells, and stacked or wound anode-separator-cathode ensembles.
One presently used design form is a prismatic lithium-ion battery having a fixed housing, for example, made up of aluminum, also referred to as a hard case. The prismatic battery having a fixed housing includes windings, also referred to as jelly rolls, which in turn include a cathode, an anode, and a separator which is impregnated with liquid lithium ion-conducting electrolyte. The anode is frequently made up of a mixture of an active graphite material, an electrically conductive additive such as conductive carbon black, and an electrode binder. The anode is generally deposited on a copper-based current collector foil. The windings are then connected to current collectors, and the complete structure is hermetically sealed in an aluminum container or a stainless-steel container. Several of such cells are assembled together with a battery management system (BMS) into a complete battery module or into a battery store.
Generally, a lithium-ion cell, which includes a transition metal oxide cathode having a layer structure made, for example, of Li1+x(Ni1/3Mn1/3Co1/3)1-xO2), and a graphite anode, is operated at 2.7 volts to 4.2 volts, which corresponds to the upper limit during charging and the lower limit during discharging. The voltage of a cell is the difference between the individual potentials of the electrodes:Voltage of the cell=potential of the cathode−potential of the anode.
Under normal operating conditions, the potential of the cathode is between 2.9 volts and 4.2 volts with respect to elemental lithium. Under normal operating conditions, the potential of the anode is between 0.05 volts and 0.8 volts with respect to elemental lithium. However, if an equilibrium in the cell is not corrected, if, for example, a ratio of a capacity of the cathode to a capacity of the anode is not optimal, the potential of the anode may increase very rapidly. This results in a decrease in the overall voltage of the cell. A state in which the potential of the anode in a lithium cell which includes transition metal oxides and graphite increases to more than 2 volts compared with elemental lithium is referred to as deep discharge.
In the case of such a high potential of the anode, a solid electrolyte interfacial layer, also referred to as a solid electrolyte interface (SEI), which normally stabilizes the graphite in the anode, is irreversibly damaged. An exposed graphite surface then again comes into direct contact with the electrolyte, which results in additional undesirable parasitic reactions. These undesirable reactions may result in loss of electrolyte in the cell, gas formation, a decrease in a reversible capacity of the cell, formation of uneven SEI layers which are relevant for safety reasons, an increase in an internal impedance of the cell and/or increased resistance of the cell, and a reduction of the service life of the battery. If the potential of the anode during a deep discharge increases up to 3.5 volts compared with elemental lithium, copper which is contained in the current collector of the anode oxidizes and generates Cu2+ ions, which dissolve in the electrolyte. These Cu2+ ions are reduced to elemental copper in a subsequent charge/discharge cycle and may lead to serious internal short circuits in the cell or in the battery, thus possibly resulting in an explosion.
Generally, the battery management system (BMS) should detect such a sudden voltage drop in the cell and prevent a deep discharge of the cell. However, a change in the voltage may occur so rapidly that the cell is in the deep discharge state before the BMS is activated in order to prevent the deep discharge. In addition, such prevention of a deep discharge depends on the proper functioning of the BMS and cannot be guaranteed if the BMS fails.
Various electrodes are described in the related art.
US 2007/0148545 A1 describes electrode materials and lithium battery systems. A material contains lithium titanate having a plurality of primary particles and secondary particles, wherein an average size of the primary particles is between 1 nm and approximately 500 nm, and an average size of the secondary particles is between approximately 1 μm and approximately 4 μm. The lithium titanate may be coated with carbon. An electrode may contain a current collector and a binder, wherein the lithium titanate is applied to the current collector.
US 2001/0076523 A1 discloses a medical device including a lithium-ion battery. The lithium-ion battery includes a positive electrode having a current collector and a first active material, a negative electrode having a current collector and a second active material, and an auxiliary electrode having a current collector and a third active material. The current collector of the negative electrode may be made of titanium or a titanium-metal alloy. The current collector of the negative electrode may include a layer of an active material which may contain lithium titanate such as Li4Ti5O12, instead of materials containing carbon. The third active material has a charge capacity and a discharge capacity below a corrosion potential of the current collector of the negative electrode and above a decomposition potential of the first active material. The auxiliary electrode may be selectively connected to the positive electrode or the negative electrode.
KR 2007-0108579 A describes a mixed material of a negative electrode, which contains composite nitrides containing lithium, wherein a discharge capacity of the composite nitrides is greater than a charge capacity of the composite nitrides, in order to prevent a discharge of a secondary battery. An active material of a negative electrode contains a composite nitride containing lithium or two or more composite nitrides containing lithium, which satisfy the formula Li3-xMxN. M corresponds to one, two, or more metals or transition metal elements selected from the group comprising Co, Ni, Fe, Cu, Zn, Cr, Cd, Zr, Mo, Ti, and V. x is greater than 0 and less than 3. Composite nitrides containing lithium have a high discharge capacity of between 0.5 volts and 3.0 volts.
In the related art, it is disadvantageous that a deep discharge is not satisfactorily prevented, an auxiliary electrode is required, or a difference between the discharge capacity and the charge capacity of the electrode material exists, so that the charge/discharge cycles are reversible only to a limited extent.